1. Field of the Invention
The present invention relates to a bottom hole assembly (BHA) for use in directional rotary drilling.
2. Description of the Related Art
Current directional drilling practice is to drill the greatest portion of a deviated well using rotary bottom hole assemblies. A rotary assembly is designed to increase (build) or decrease (drop) the inclination in the vertical plane, and, to a lesser extent, to turn to the right or the left in the horizontal plane. However, the degree to which the rotary assembly will deviate cannot be accurately preset. Furthermore, even if a slight change in the deviation trajectory becomes necessary while drilling, the BHA needs to be pulled up to the surface and manually reconfigured. It is for these reasons that in sections of a well where a reliable, accurate, or continuous change in the rate of deviation is required, the rotary assembly is brought to the surface and a steerable assembly is put in its place. A steerable assembly is one which includes a downhole motor and a bent sub near to the bit. In such a case the BHA comprises a drill bit, a bent sub which angles the bit axis at around 1/2.degree.-3.degree. from the drillstring axis, and a downhole motor connected to the bit. The new path of the borehole is achieved by turning the drillstring until the bit is pointing in the desired direction due to the bent sub. This position can be found by means of instruments located in the BHA, such as accelerometers or magnetometers, which can determine the direction in which the bit is facing and transmit the information to the surface. Once the bit is oriented in the appropriate direction, it is rotated by means of the downhole motor, weight being applied to the bit from the surface in the usual manner but without rotation of the drillstring, and so is allowed to drill ahead in the desired direction. Once the trajectory of the borehole has deviated to the required degree, the drillstring is pulled from the well and the BHA replaced with a rotary BHA, and rotary drilling recommences to drill straight ahead or deviate in the conventional manner.
There are a number of problems when drilling with a BHA including a bent sub and downhole motor. When drilling is conducted without rotation using the downhole motor, the rate of penetration is greatly reduced. There is also a greater likelihood that the drillstring will become stock. Furthermore, use of a combination of rotary and steerable tools often requires a greater number of trips of the drillstring out of the hole to change or adjust BHA components; this combined with the slower rate of penetration, can seriously affect the rate of progress of a well and add to the overall cost, as can the cost of the BHA equipment used. For this reason, together with the problems outlined above, alternative methods of directional drilling have been sought. U.S. Pat. Nos. 4,597,455 and 4,732,223 describe a downhole adjustable sub which can provide either a bent or straight BHA according to requirements, the sub being activated by dropping a ball through the drillstring into the sub so as to activate a clutch mechanism. U.S. Pat. No. 4,739,843 describes a system in which an eccentric stabilizer which is prevented from rotating is used to create the deviation at the bit, and a flexible section of drill pipe allows a tight radius of curvature to be made by the bit while rotary drilling.
An alternative approach to directional rotary drilling is described in U.S. Pat. No. 4,995,465 and GB 2,246,151, in which either an asymmetric bit or a normal bit and bent sub combination is used in rotary drilling in conjunction with some means for creating pulses in the weight applied to the bit. By timing the weight on bit (WOB) pulses to coincide with a given rotary position of the bit, a deviation can be created.
The present invention resides in the realization that the BHA design can be optimized so as to maximize the deviation that can be effectively achieved with periodic modification of the cutting action of the bit in the borehole.